Tunable optical bandpass filters offer significant advantages in dense wavelength division multiplexing or DWDM optical networks. Single channel tunable bandpass filters are used for amplified spontaneous emission suppression after optical amplification, and in receivers as adaptive pre-filters for noise reduction. When used for noise reduction, a fiber-in, fiber-out filter followed by a separate fiber-coupled receiver may be used, or the filter and detector may be integrated into a single tunable receiver unit. Universal line cards based on tunable receivers may reduce the costs of maintaining inventory and spares. Tunable filters may also reduce costs for optical performance monitoring by allowing one monitor to select between multiple channels. More generally, tunable filters can be used in reconfigurable optical add-drop multiplexers (ROADM) which are often the central switching element of a transparent optical node.
Commercially available tunable filter technologies include, but are not limited to, fiber Bragg gratings, arrayed waveguide gratings, linearly variable thin film dielectric filters, Mach-Zehender interferometers, fiber Fabry-Perot etalons, Fabry-Perot etalons with deformable semiconductor multi-layer mirrors, and certain devices combining two or more or these elements. See, for example, “Tunable Optical Filters for Dense WDM Networks,” by Dan Sadot and Effraim Bolmovich, IEEE Communications Magazine, December 1998, pp. 50–55. These and other filter technologies have certain drawbacks that limit or reduce their desirability. For example, some of these devices suffer from slow tuning speed, large form factor, large power consumption, narrow tuning range, large insertion loss, repeating passbands, and/or poor adjacent channel isolation. Additionally, in some such devices the filter bandpass shape cannot be easily modified to range from a broad flat-top to a narrow Gaussian. The optimum filter for a given application may require tailoring the bandpass shape or it may depend on the ease with which a receiver can be integrated into the device. See, for example, “A quantum-limited, optically-matched communication link, D. D. Caplan and W. A. Alter, paper MM2-1, Proceedings of the Optical Fiber Communication Conference, OSA Technical Digest Series, Optical Society of America, Washington, D.C., 2001. Further, in certain of such devices the center wavelength may be adjusted by a voltage-controlled position with no internal wavelength reference, and may require complex temperature mapping. In addition, for some of such devices it may be difficult, if not impossible, to construct as a combined filter and receiver.
Fixed diffraction gratings typically have not been used in telecommunications-grade tunable filter applications. Tunable filters incorporating fixed diffraction gratings are commonly multi-element devices that utilize electronically-driven deflection elements in combination with fixed gratings to generate a narrow and tunable transmission function. See, for example, U.S. Pat. Nos. 5,946,128 and 6,141,361. Rotating diffraction gratings, mounted in one of several well-known scanning monochrometer configurations, may be used for spectrum analysis but, generally, are not suitable for telecom-grade tunable filter applications due to their mechanical complexity and size.
As such, there is a need for a diffractive tunable filter that does not include many of the foregoing disadvantages and which desirably provides superior optical transmission and tuning characteristics.